An Alabama woman was charged with capital murder Wednesday after she walked into a police station and confessed to suffocating her 4-year-old daughter with a pillow last summer.

Stephanie Diane Smith, also known as Stephanie Diane Cooper, 25, of Athens, is charged with capital murder in the death of Zadie Wren Cooper. Zadie died at Huntsville Hospital on July 7, 2016, three days after Smith called 911 and reported that her daughter was not breathing.

Athens police officials said that Smith told investigators at the time that she heard a noise on the baby monitor July 4 and that when she went to check on Zadie, she found her wedged between her bed and the wall, not breathing. When police and emergency personnel arrived at Smith’s apartment, they found her performing CPR with the help of a dispatcher.

Zadie was on life support for three days, until doctors’ tests found that she had no brain activity. Her family removed her from life support and she died.

Athens Police Chief Floyd Johnson said during a news conference on Thursday that a private autopsy requested by the girl’s family could not find the cause of her death. Investigators were awaiting medical and forensic reports that they hoped would provide clues to what happened to the toddler.

The case remained open but was at a standstill until Smith walked into the station Wednesday and said she wanted to speak to an investigator about her daughter’s death. In her confession, Smith told police -- who later confirmed it with her family -- that she has a long history of mental illness.

“(She) told us that she has mental issues and, at the time she had some issues and she was not on her medication,” a somber Johnson said during the news conference, livestreamed on Facebook by WHNT News 19 in Huntsville.

“She went into Zadie’s room, put a pillow over her head; her face -- she was sleeping on her back – and smothered the child,” Johnson said. “A short time later, she realized what was going on; she removed the pillow. Zadie was not breathing, and she called 911.”

Smith said she initially lied about what happened to police and Department of Human Resources officials because she was afraid of what would happen to her, Johnson said.

Because of the child’s age, Smith was booked on a charge of capital murder. She is being held in the Limestone County Jail.

Johnson said it is the first time in his long career that he’s had someone walk into the station on their own and confess to murder. He said it appeared that Smith’s conscience drove her to tell the truth about her daughter’s death.

“Yesterday, it was very clear (that) she was in a very good frame of mind,” Johnson said. “She knew exactly what she was doing. She was talking with us. She wanted to get this out, that she had caused this.”

“I’ve not seen one like this, where they come in and straight-up tell you,” he said.

He said Smith reported having numerous mental health issues and listed multiple prescribed drugs that she had stopped taking at the time of her daughter’s death. She said she had not taken her medication for three days at the time she suffocated Zadie.

Johnson declined to name the conditions for reporters.

“I’m not really qualified to make those statements, and I don’t have any paperwork at this time telling me exactly what all those could possibly be,” Johnson said. “And that would be an issue the courts would have to deal with.”

A post on Smith’s Instagram account, a photo of 13 pills, has the caption: “This is what it takes for me to barely function.” It includes hashtags for bipolar disorder, borderline personality disorder, post-traumatic stress disorder and anxiety.

Smith told investigators she’s experienced similar but less severe incidents in the past.

“She said she ‘sees red,’” Johnson said. “And when she sees red, she doesn’t know what happens until it goes away.”

It was not immediately clear if Smith harmed her daughter or anyone else in the previous incidents.

Johnson, who paused to gather his thoughts several times during his statements, indicated that the interview with Smith was an emotional one for all involved.

“The first thing you think of is Zadie,” Johnson said. “And then you start thinking of the family and the pain that the actions have caused. You think about what Stephanie’s going through, you think about the child’s father, you think about the grandparents, you think about friends.”

He said that the toddler’s family, as well as her mother’s, were very upset to learn about the confession. He said it was also readily apparent during her interview with investigators that Smith was also hurting in the aftermath of her daughter’s death.

Several photos of the smiling toddler were posted to Smith’s Instagram account, listed under Stephanie Cooper, including one of the two of them together that she said was taken on a day when she wasn’t feeling well and her daughter made her feel better.

“Lately I haven't been well. I need a dose of you, but you're out of reach. I hope you knew how incredibly important you were to me. You were and still are my world,” Smith wrote. “I'm lost without you. Completely lost. I need you, but you're not here, and I'm so sorry I couldn't do anything to stop that.”

Johnson also revealed that shortly before Smith walked into the station, she texted five of her friends and confessed to the slaying. One of those friends came forward with the information, Johnson said.

The chief said that the death of any child automatically raises red flags for police but that the circumstances of Zadie’s death made his investigators suspicious from the beginning. They just could not prove one way or another what happened to the toddler, he said.

Smith, who told investigators she feared she might hurt herself if she didn’t admit what she’d done, is being closely monitored by jail staff, Johnson said.